The present invention generally relates to a process for producing cup tappets for reciprocating-piston machines, and more particularly to a process for reliably preventing cracks in cup tappets.
The production process for tappets which is disclosed by German Published, Unexamined Patent Application (DE-OS) 2,526,656 is intended primarily for those tappets in which the stroke movement is indirectly transmitted to the valve stem of gas-exchange valves via interposed tappet rods and rockers. In this production process, a pressure plate and a cup casing are welded together by electron-beam welding. To secure the mutual positioning of the two individual parts when they are still not welded together, a small peg or a small shoulder, onto which the cup casing is attached in a centered manner, is lathe-turned on the pressure plate.
German Published, Unexamined Patent Application (DE-OS) 3,540,882 shows a cup tappet, in which the stroke movement is transmitted to the valve stem directly, that is to say without interposed tappet rods or rockers. A hydraulic play-compensating element is incorporated in the cup tappet. The pressure plate is made of a wear-resistant tool steel and has an axially extending skirt which forms part of the outer surface of the cup casing on its outer edge. The pressure-plate blank is therefore essentially a bowl-shaped structure. The remaining parts of the cup tappet are produced from cold-extrudable steel and are connected to the bowl-shaped pressure plate by a laser welding seam located in the casing region in an approximately central axial position.
A disadvantage of the two above-noted production processes is that the welding processes used are highly conducive to the formation of cracks especially in the material of the pressure plate. In particular, there is a pronounced quenching effect attributable to the unheated material and therefore a local embrittlement of the material in the welding region, because of the narrowly localized heat input. This, despite the relatively low heat distortions, leads to cracks. Moreover, because of the high vacuum necessary for electron-beam welding, this welding process is very time-consuming and does not allow short cycle times. Finally, the necessary shaping of the pressure plates can be carried out only in a chip-removing manner, a shaping process involving a relatively high outlay. Furthermore, material-cutting shaping processes entail a relatively high loss of material.
Accordingly, an object of the present invention is to develop further the relevant process for the production of cup tappets, while, at the same time, maintaining, if not even reducing the cycle time, to the effect that cracks in the cup tappets can be reliably prevented.
According to preferred embodiments of the present invention, these and other objects are achieved utilizing a friction-welding Process, which is known per se and which works in a very economical way to provide a crack-free welded joint between the cup casing and pressure plate forming the cup tappet. A carbonization of the cup casing ensures an approximately uniform frictional heating despite greatly differing material properties and material cross sections between the two individual parts forming the cup tappet.
The crack-free welded joint provided as a result of carbonization is especially unexpected in that carbon contents above only approximately 0.25% can lead to the formation of cracks during welding. Surprisingly, however, this is not to be observed in the particular use described here.
Because the friction-welding process is adopted, the pressure-plate blank can also advantageously be designed as a simple flat round, thereby considerably reducing the costs for the production of the pressure-plate blanks as well as minimizing resulting waste material. The individual advantageous features of preferred embodiments of the present invention interact to lead to a common outcome in which cup tappets can be produced in a crack-free and economical manner.
In particular, with the process according to advantageous features of preferred embodiments of the present invention, it is also possible to produce cup tappets with two welding seams arranged concentrically one in the other, as are desirable, under certain circumstances, for cup tappets with an incorporated play-compensating element.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.